ISPConfig network IP address settings - public or local?

Which IP address I should have on ISPConfig setings
System / Server Config / Server / IP address

Should it be my public-IP (from ISP) or my internal-IP (static inhouse-IP like 192.168.xx.yyy)?
(In my case Server1 is behind ADSL-modem (as DMZ as seems to be well protected by perfect server instructions )

All seems to be working fine, but just to get it right, please let me know.